A conversion system of the aforementioned type is known. FIG. 1 illustrates such a conversion system.
In FIG. 1, the conversion system 10 is a voltage inverter with five output levels, and comprises two input terminals 12, four capacitors 14 connected in series between the input terminals 12 and connected to one another in pairs via a respective middle point 16. The conversion system 10 also comprises, for the or each phase of the output voltage, an output terminal 18 and two switching branches 20 each connected between the output terminal 18 and a respective input terminal 12. Each switching branch comprises four switching cells 22 connected in series and three intermediate points 24, the switching cells 22 being successively connected to one another by a corresponding intermediate point 24.
The conversion system 10 also includes three intermediate branches 26 each connected to a respective middle point 16 and comprising two diodes 28, 30. The first diode 28 is directly connected between said respective middle point 16 and a corresponding intermediate point of one of the two switching branches 20, the second diode 30 being inversely connected between said respective middle point 16 and an intermediate point 24 of the other of the two switching branches 20. The conversion system 10 also includes a means, not shown, for controlling the switching cells 22.
Such a conversion system is likely to involve electric charge imbalances at the middle points 16, which then causes a dissymmetry of the voltage and current at the output of the conversion system. The conversion system then generally includes means for balancing the voltage at the terminals of each of the capacitors, such as equilibration means by adding a homopolar component with a shared mode on each of the phases of the alternating output current, the shift between the phases making it possible to charge or discharge the capacitors.
However, such equilibration means are effective when the amplitude of the alternating voltage is below a threshold value substantially equal to one quarter of the value of the direct input voltage, and do not allow correct equilibration when the amplitude of the alternating output current is above said aforementioned value. Furthermore, such equilibration means cause excess costs.
The aim of the invention is to propose a conversion system making it possible to reduce the voltage imbalance at middle points irrespective of the voltage and current delivered by the conversion system, while not requiring additional means for balancing the voltage at the terminals of the capacitors.